Portable electronic devices such as cell phones, tablet computers, MP3 music players, and the like typically derive electrical power either from internal batteries or from external power sources. The external power source may be a computer or other electronic appliance with which the portable device communicates. Or the external power source may be a dedicated charging power supply.
Many portable devices use USB ports both to communicate with external appliances and to receive electrical power. This power may be used to operate the device, to recharging the batteries, or both. An external appliance meeting the USB 2.0 standard can provide up to 2.5 watts (500 milliamps at 5 volts) through a USB port. Dedicated charging power supplies on the other hand, may provide up to 10 watts (2 amps at 5 volts) or more.
USB battery charging specification revision 1.1, dated 15 Apr. 2009, describes a configuration that provides up to 1.5 amps (7.5 watts) through a single USB port of an appliance. This specification also describes a handshaking protocol whereby a portable device and an appliance communicate to determine how much electrical power the appliance can provide. Very few computers or other appliances have implemented revision 1.1, and hence most appliances are limited to providing 2.5 watts through a USB port.